For continuously measuring the fill level of liquids and solids in containers, utilising measurement of the run time of electromagnetic waves, measuring devices are usually installed on or in the ceiling of the container, wherein said measuring devices subsequently transmit waves, either guided through a waveguide or radiated by way of an antenna, in the direction of the product contained in the container. The waves reflected by the product contained in the container are subsequently received by the measuring device. From the measured run time the distance between the sensor and the product contained in the container can be derived, while from the knowledge of the position of the sensor from the bottom of the container the sought fill level can be obtained.
Various methods are known in order to carry out run time measurements of these devices, which when the wave is radiated by way of an antenna is referred to as a radar fill level sensor, and when the wave is guided is often referred to as a TDR fill level sensor (time domain reflectometry). The two most-often used methods are the FMCW method and the pulse radar method. In the FMCW method, the transit time results from a measured differential frequency between the transmitted and the received continuous high frequency signal that has been frequency modulated in a linear manner.
In the pulse radar, short high-frequency pulses are emitted which are received after the corresponding run time. The time that has passed in between has to be determined as precisely as possible. Since the waves propagate almost at the speed of light the times to be measured are correspondingly short so that normally circuit arrangements are used which by way of sequential sampling of the receive signal convert said receive signal to a slowed down image that is true to the original. This method, which is often referred to as the ETS (equivalent time sampling) method, is described in DE 31 07 444. There, as well as in specification DE 029 81 5069 U1, circuit designs are described with which the desired slow-down in time can be achieved. The sampling method used is based on a sampling signal which from each receive signal triggered by a transmission pulse generates only one brief sampling value. If one controls the position in time of the sampling values relative to the transmit signal or receive signal so that a continuous linear increase in the sampling time between transmit pulses results, then the individual sampling values placed one behind the other result in the desired slowed down receive signal. In this process the amount of the increase in the sampling time relative to the transmit pulse determines the extent of the time expanding.
Two methods are known that cause the required linear increase in the sampling time. One method is characterised by an oscillator or a clock pulse control circuit with a adjustable downstream delay circuit. The pulse generated by the clock pulse signal source triggers not only emission of the transmit pulses, but also, delayed by way of the adjustable delay circuit, generation of the sampling signal. U.S. Pat. No. 5,563,605 describes one implementation of this method.
The second method for implementing the linear increase in the sampling time comprises two oscillators whose frequencies differ slightly. From one oscillator, clock-pulse flanks for triggering the transmit pulse are derived, while from the other oscillator clock-pulse flanks for triggering the sampling signal are obtained. As a result of the slight frequency difference, wherein the sampling repetition frequency is preferably somewhat lower than the transmit repeat frequency, the point in time of sampling shifts, relative to the point in time of transmission, in a linear manner from one transmission period to the next. As long as the frequency difference of the two oscillators is kept constant, high linearity of the time shift and thus high measuring accuracy can be achieved. For this reason one of the two oscillators is designed such that it can be varied via a frequency by way of a control input. The frequency of this oscillator is regulated such that a frequency difference of the two oscillators corresponds to a desired value to be specified. In this arrangement the ratio of transmit repeat frequency to frequency difference determines the time expanding factor of the sampling method.
DE 101 06 681 discloses the formation of the frequency difference by means of a digital phase detector. Usually a signal that most often is a square-wave signal whose frequency corresponds to the difference between the two oscillator frequencies results from forming the frequency difference. Measuring and comparing this difference with a specified desired value makes it possible to regulate one of the oscillators. In a simple manner this can largely be handled by a microcontroller. In this way it also becomes possible, by way of software-based definition of various desired values, to set different time expanding factors for adapting the sensor to variable measuring conditions. However, the smaller the frequency difference set, the more problematical exact regulation of the frequency difference, because measuring the differential frequency requires the time duration corresponding to a full period, for example from one rising flank of the square-wave signal to the next one, so that the space in time in which regulation can become active becomes increasingly large as the differential frequency decreases.